This invention relates to a gas sensitive unit for use in a gas sensor in detecting a gas, such as methane, ethane, propane, normal butane, isobutane, a mixture thereof, carbon monoxide, or town gas. The gas sensor indicates the concentration of the gas in the surrounding atmosphere and alarms leakage of such a gas.
A conventional gas sensitive unit comprises a gas sensitive semiconductor element, which has an electrical conductivity between 10.sup.3 and 10.sup.-10 mho/cm at normal temperature. Typical semiconductor material is stannic oxide (SnO.sub.2) and gamma-ferric oxide (.gamma.-Fe.sub.2 O.sub.3). The conductivity generally grows greater with the ambient temperature and an increase in the concentration of the gas in the surrounding atmosphere. As described in U.S. Pat. No. 3,695,848 issued to Naoyoshi Taguchi, the semiconductor material has a substantially linear characteristic versus the gas concentration when heated to about 150.degree.-250.degree. C. The gas sensitive unit has therefore been manufactured as a combination of the semiconductor element and a heating wire.
For use as the gas sensor, it is necessary to combine an electrical circuit with the gas sensitive element. Inasmuch as the gas sensitive unit has been manufactured as a combination, it has not been tried to manufacture the gas sensor as an integrated circuit. As a result, a gas sensor has been bulky. It has been difficult to manufacture the gas sensor in mass production.
When a bridge circuit is used as the electrical circuit, the combination of the gas sensitive semiconductor element and the heating wire has made it necessary to manually adjust the bridge. As a result of the troublesome manual adjustment, the gas sensor has been defective in the sensitivity and the stability.